Processes for producing polymeric foams are well known. DE-A-102 33 703 relates to such a process for producing nanocellular polymeric foams having cell sizes of 1 μm or less wherein a polymer is dissolved in a sublimable solvent, the polymer solution is cooled within not more than two seconds to a temperature below the melting point of the solvent to obtain a frozen mixture and the frozen solvent is removed by freeze drying to obtain a nanocellular polymeric foam.
Polyamides are mentioned as well as numerous polymers, while the examples concern polystyrene.
U.S. Pat. No. 5,300,272 relates to the production of microcellular carbon foam having open porosity. The precursor for the carbon foam is prepared by the phase inversion of a polyacrylonitrile in a solution consisting essentially of an alkali metal halide and a phase inversion solvent for the polyacrylonitrile. Propylene carbonate in particular is used as solvent.
J. H. Aubert and R. L. Clough in Polymer, 1985, vol. 26, pages 2047 to 2054, describe the production of polystyrene foams having a microporous open-cell structure. The polystyrene is dissolved in a solvent such as cyclohexane, then the solution is devolatilized and thereafter rapidly cooled, freezing the solvent. The solvent is then removed by freeze drying to leave the polymer behind as a foam.
U.S. Pat. No. 4,118,449 describes the production of microcellular cellulose acetate foam wherein cellulose acetate is dissolved in an acetone-based solvent, the solution obtained is converted into a gel in a water bath by cooling, and the gel is freeze dried, which causes the water and the solvent to sublime and the gel structure to solidify into a microcellular foam.
DE-A-40 09 865 describes the production of hollow polyacrylonitrile fibers for membrane applications that have an asymmetrical pore structure in the sheath. A polyacrylonitrile solution in DMF is extruded together with glycerol as core liquid, and then precipitated in water to form the hollow filament.
It is known from US-A-2005 0288484 to use ionic liquids as solvents for different polymers to precipitate polymer blends by subsequent addition of a nonsolvent. The blends are based on cellulose.
Porous polymeric structures are required for a multiplicity of uses, for example in filter and membrane technology and also in the medical sector. The need is frequently for ideally uniform microporous, open-cell foams, particularly in the form of fibers.